After transcription, mRNA undergoes several crucial processing steps to become ready for translation. Initially, a 5' cap, composed of a methylguanosine molecule, is added to the mRNA. This cap serves to protect the RNA from degradation and plays a vital role in the translation process.
Next, the mRNA receives a polyadenylation tail at the 3' end, typically consisting of 150 to 200 adenine nucleotides. The addition of this tail is triggered by a specific polyadenylation signal, represented by the sequence AAUAAA, located at the end of the transcript. The poly A tail is essential for the export of mRNA from the nucleus, facilitating its translation into protein.
Another significant step in mRNA processing is splicing, which involves the removal of non-coding segments known as introns from the coding segments called exons. The spliceosome, a complex of proteins and small nuclear RNAs (snRNAs), is responsible for this process. It recognizes specific sequences at the splice sites: a 5' splice site marked by the GU nucleotide, a 3' splice site marked by the AG nucleotide, and a branch point, which is a single adenine nucleotide located upstream of the 3' splice site. During splicing, the spliceosome forms a lariat structure by cutting at the GU site, excising the intron, and joining the exons together to create a mature mRNA transcript.
Alternative splicing can occur, allowing for different combinations of exons to be joined, which contributes to genetic diversity by producing multiple protein variants from a single gene.
Finally, RNA editing is another form of post-transcriptional modification, though it is more common in prokaryotes and some lower eukaryotes. This process involves altering the RNA sequence through substitution, insertion, or deletion of nucleotides. Guide RNAs play a crucial role in RNA editing by directing where these modifications should occur, ensuring that the changes are not random but rather specific to enhance genetic diversity.
